On June 28, 1914, Archduke Franz Ferdinand of Austria, the heir to the Austro-Hungarian throne, and his wife Sophie, Duchess of Hohenberg, were assassinated in Sarajevo, the capital of Bosnia-Herzegovina (formally annexed by Austria-Hungary in 1908). The assassin, Gavrilo Princip, was a Bosnian Serb nationalist and member of the Black Hand, a secret society dedicated to the unification of all South Slavic peoples into a single state.
The assassination sparked a diplomatic crisis between Austria-Hungary and Serbia. Austria-Hungary, with the backing of Germany, issued an ultimatum to Serbia demanding that it crack down on anti-Austrian activities and punish those responsible for the assassination. Serbia accepted most of the demands but rejected one that would have allowed Austrian police to operate on Serbian soil.
On July 28, 1914, Austria-Hungary declared war on Serbia, marking the beginning of World War I. The conflict quickly spread to other European powers, with Germany, France, Russia, the United Kingdom, and Italy all entering the war. The war lasted for four years and resulted in the deaths of millions of people.